1. Field of the Invention
The present invention concerns an optical beam splitter which is necessary in cases where, for example, halftone plate duplicate images are recorded on a recording material by controlling a light-exposure means on the recording side in accordance with image signals obtained by the photoelectric manipulation of an original image, and especially in cases where halftone plate images are recorded by independently modulating a multiple number of beams on the basis of image signals. The present invention also concerns an optical beam splitter which can be used for the optical beam splitting required in cases where halftone images are cut into plastic by means of a laser working device.
2. Prior Art
The recording of halftone plate images by the relative scanning of a multiple number of light beams (lined up in a row) across the surface of a recording material, with the light beams being independently modulated on the basis of image signals, has been conventionally known in the art. In most cases, the multiple number of light beams are obtained by installing a multiple number of totally reflective mirrors and semi-reflective mirrors, and splitting a single light beam enerated by an argon laser by reflecting the light beam from the mirrors. The respective light beams thus obtained are independently modulated by a multi-channel ultrasonic modulator and are then reduced in diameter by a crystal optical system and directed onto the surface of a recording material.
Inventions, which use a single optical beam splitter, and in which a single light beam is split into a multiple number of light beams (lined up in a row) by repeated internal reflection in this optical beam splitter, are described in Japanese Patent Application Laid-Open (Kokai) No. 52-122135 and Japanese Patent Application Laid-Open (Kokai) No. 58-10713. In both of these inventions, one surface of a single glass plate is coated with a totally reflective film, while the other surface is coated in different regions with semi-reflective films having different reflectivities. To describe the semi-reflective films in detail, e. g. in a case where a single light beam is split into ten (10) light beams, the first region is coated with a semi-reflective film which has a reflectivity of 9/10 (i. e., a transmissivity of 1/10). The next region is coated with a semi-reflective film which has a reflectivity of 8/9 (i. e., a transmissivity of 1/9), and the next region after this is coated with a semi-reflective film which has a reflectivity of 7/8 (i. e., a transmissivity of 1/8 ). In this way, successive regions are coated with semi-reflective films whose reflectivities gradually decrease in a numerical series, so that the last region is coated with a semi-reflective film which has a reflectivity of 1/2 (i. e., a transmissivity of 1/2).
However, if a single laser beam generated by an argon laser is converted into a multiple number of laser beams by the installation of mirrors or optical fibers, the apparatus requires a large amount of space. Furthermore, adjustment of the mirrors or optical fibers requires a high degree of technical skill and a great amount of time. Accordingly, such techniques involve considerable trouble.
On the other hand, the abovementioned defects are completely eliminated by the optical beam splitters described in Japanese Patent Application Laid-Open (Kokai) No. 58-10713 and Japanese Patent Application Laid-Open (Kokai) No. 52-122135.
In both of these optical beam splitters, however, the splitting of a single light beam into 10 light beams requires an arrangement in which nine regions with an extremely narrow width of 2 mm or less are coated with semi-reflective films whose reflectivities vary according to the region as described above. Since the failure of even one coating is impermissible, very difficult techniques are required.
The optical beam splitter described in Japanese Patent Application Laid-Open (Kokai) No. 52-122135 is advantageous in that an increase in the number of split light beams does not require any increase in the size of the crystal optical system. However, since the split light beams are not parallel to each other, the modulating efficiency of the modulator drops, and light leakage may occur. Furthermore, formation of the acoustic electrodes of the modulator is also difficult. In addition, maintaining the thickness and angle of intersection of the two surfaces of the optical beam splitter at prescribed values with an ultra-high degree of precision is difficult, and the manufacture of the optical beam splitter involves great difficulty. If the thickness or angle of intersection of the two surfaces of the optical beam splitter shows even a slight variation, the focal distances of the respective light beams will show a large variation, and the spacing between the optical beam splitter and the modulator, as well as the spacing between the modulator and a "crystal optical system modulator," will be unavoidably different in each individual apparatus.